At present, it is multiple-deck screening machines that are most widely used in practice, wherein the separation of polydisperse materials according to size is accomplished by letting the material pass through a number of screens with meshes of different sizes, which are placed one beneath the other. In this case, to provide an efficient separation of the material, an individual optimum amplitude of vibration should be assigned to each screen, depending on manufacturing parameters of which classification size is the most important one.
Known in the prior art is a vibratory screening machine of WA series manufactured by "Rhewun" of the Federal Republic of Germany (Aufbereitungs-Technik, Nr. 7, July, 1977, (G. Erlenstadt, "Schallsiebmaschinen-Weiterentwicklung und neue Betriebsergebnisse", p. 333-336), comprising a frame, screens with meshes of different sizes positioned in the frame one above the other, pushers formed by impact levers located beneath the screen, each of the levers being mounted on a supporting shaft and adapted to make contact with the surface of the screen, and electromagnetic vibrating drives. Each electromagnetic vibrating drive is coupled to one supporting shaft for reversible rotation thereof through a given angle. In so doing, the impact lever secured to the shaft transmits vibrations of a particular amplitude from the vibrating drive to the underlying screen.
One disadvantage, however, of this device is its high cost, due to a plurality of expensive electromagnetic vibrating drives, their number increasing with the number of screens in the machine.
Known in the art is a vibratory screening machine (DE, C, 1239919) comprising a frame, at least two screens located in the frame one above the other, pushers formed by double-arm levers placed in the interscreen space, each of them being rigidly secured to a shaft and adapted to make contact with different screens, and a vibrating drive. The vibrating drive is coupled to one of the shafts for reversible rotation thereof through a given angle, while the remaining shafts are joined both to the aforementioned shaft and to one another by means of kinematic transmissions. Each lever type pusher has arms of the same length arranged symmetrically about the shaft axis. Said pushers transmit vibrations of a particular amplitude from the vibrating drive to the screens.
In such a device the vibrations imparted to the screens are of the same amplitude determined by the vibration amplitude of the drive. Such vibrations of the screens fail to provide the required screening effect, since the materials on individual screens are subjected to different processing conditions.